Typical radio frequency (RF) power transistors are based on field effect transistor (FET) technology, such as a high electron mobility transistor (HEMT), although some RF power transistors are based on bipolar junction technology. In the case of FET or HEMT technology, a typical RF power transistor includes a plurality of cells connected in parallel. Each cell includes a source, gate, and drain. The cells are typically formed on top of one or more epitaxial layers grown above a non-electrically-conductive or relatively high resistivity substrate (e.g., 5000 to 10,000 ohm-cm).
The typical RF power FET or HEMT further includes an input signal metallization pad disposed over the epitaxial layers. The input signal metallization pad is electrically connected to the gates of the cells. Similarly, the typical RF power FET or HEMT also includes an output signal metallization pad disposed over the high resistivity substrate. In a similar manner, the output signal metallization pad is electrically connected to the drains of the cells.
The sources of the cells are electrically connected to a ground terminal, which, in many cases, is at a metallization layer disposed on the bottom surface of the high resistivity substrate. In order to effectuate the grounding of the sources, metalized via holes are formed through the one or more epitaxial layers and the high resistivity substrate to electrically connect the sources to the grounded bottom metallization layer of the substrate, respectively.
There are many drawbacks associated with the typical RF power transistor, and in particular, to the grounding of the source of each cell of the transistor. More specifically, each metalized via hole used for electrically connecting a corresponding source to the grounded metallization layer on the bottom of the high resistivity substrate has an inherent resistance and inductance. The resistance and inductance are directly related to the length of the metalized via hole, and inversely related to the diameter of the metalized via hole.
Such resistance produces RF losses through the metalized via holes, which results in an effective lower gain for the transistor, as well as a lower saturation point, lower 3rd-order intercept, higher output harmonics, and other adverse effects. Moreover, the inductances of the metalized via holes lowers the high frequency performance of the transistor. These adverse effects also reduce the effectiveness of the input RF signal combining as well as the output RF power signal extraction. Accordingly, it is generally desirable to reduce the resistance and inductance associated with the RF grounding of the sources of the RF power transistor.